1. Field of the invention
The present invention concerns a method of winding an electrical coil.
The invention applies more particularly to transformer windings, especially high-voltage windings.
2. Description of the prior art
It is known to wind oblique layers with flattened turns perpendicular to the winding axis. As compared with a conventional coil with horizontal layers of contiguous helical turns, this reduces the maximum voltage between turns of successive layers and therefore makes it possible to reduce the thickness of the insulation between layers and in some instances to eliminate it entirely.
FIG. 4 of French patent 717 497 and the relevant text from page 5, line 82 through page 6, line 84 show and describe a coil with 70 turns in 20 oblique layers. The first layer comprises only one turn, the second layer comprises two turns, the third layer also comprises two turns, the fourth layer comprises three turns, the fifth layer comprises four turns, the sixth layer comprises four turns, the seventh through fourteenth layers each comprise five turns, the fifteenth layer comprises four turns, the sixteenth and seventeenth layers each comprise three turns, the eighteenth layer comprises two turns and the nineteenth and twentieth layers each comprise one turn. The layers are at 30.degree. to the horizontal.
These 70 turns wound in 20 oblique layers form five horizontal layers as can be seen in the aforementioned FIG. 4.
Numbering the turns in consecutive winding order it is found that the inner horizontal layer comprises turns 1, 2, 5, 6, 12, 13, 21, 22, 31, 32, 41, 42, 51 and 52 and that the outer horizontal layer comprises turns 17, 26, 27, 36, 37, 46, 47, 56, 57, 63, 64, 68, 69 and 70.
A first drawback of this coil is that it requires lateral flanges to retain the turns.
A second drawback results from the fact that, as noted above, the turns of the outer horizontal layer are not consecutive. As a result, if voltage adjusting tape are required, it is not usually possible to obtain a precise adjustment accurate to the voltage of one turn. The adjustment is usually performed in the following manner. If adjustment is required to a voltage 2.5% lower than the total nominal voltage, two taps are formed on the outer turns, the number of turns between the two taps having to be that required to obtain the 2.5% lower voltage. The adjustment is made by bridging between the two taps each connected to a connector. It is therefore clear that in an oblique layer coil as described in the document cited above the voltage cannot be adjusted accurate to the voltage of one turn because the turns of the outer horizontal layer are not consecutive.
The first drawback of this coil is eliminated by the document CH-A-254 093 which describes an oblique layer winding whose half-section in a plane through its axis is trapezium-shaped. The coil thus forms a stable assembly requiring no flanges. To make the coil the first step is to execute a winding with a triangular cross-section.
However, the second above drawback relating to the adjustment tape is not eliminated.
An object of the present invention is to alleviate this drawback.